Kinematic signatures of prey capture from archival tags reveal sex differences in killer whale foraging activity

Abstract

Studies of odontocete foraging ecology have been limited by the challenges of observing prey capture events and outcomes underwater. We sought to determine whether subsurface movement behavior recorded from archival tags could accurately identify foraging events by fish-eating killer whales. We used multisensor bio-logging tags attached by suction cups to Southern Resident killer whales (Orcinus orca) to: (1) identify a stereotyped movement signature that co-occurred with visually confirmed prey capture dives; (2) construct a prey capture dive detector and validate it against acoustically confirmed prey capture dives; and (3) demonstrate the utility of the detector by testing hypotheses about foraging ecology. Predation events were significantly predicted by peaks in the rate of change of acceleration (‘jerk peak’), roll angle and heading variance. Detection of prey capture dives by movement signatures enabled substantially more dives to be included in subsequent analyses compared with previous surface or acoustic detection methods. Males made significantly more prey capture dives than females and more dives to the depth of their preferred prey, Chinook salmon. Additionally, only half of the tag deployments on females (5 out of 10) included a prey capture dive, whereas all tag deployments on males exhibited at least one prey capture dive (12 out of 12). This dual approach of kinematic detection of prey capture coupled with hypothesis testing can be applied across odontocetes and other marine predators to investigate the impacts of social, environmental and anthropogenic factors on foraging ecology.

Hidden Markov models reveal temporal patterns and sex differences in killer whale behavior

“…Additionally, state 1 dives had the greatest duration. Consistent with previous results 19 , state 1 is likely associated with close pursuit and capture attempts of salmonid prey, whereby prey are chased at depths typically greater than 30 m, producing substantial jerk movement due to abrupt acceleration and deceleration, as well as changes in musculature in the head region at prey interception 29–33 . Additionally, state 1 is associated with large values of roll, and large heading variance due to repeated direction changes 34 while closing in on prey.…”

“…35,46,64,65 . The second explanation for the paucity of state 1 persistence may be that a prey item is often captured during a deep foraging dive 19 . Upon capturing a prey item, it is then typically shared among pod-members near the surface 15 , precluding the occurrence of consecutive deep dives.…”

“…Surface observations revealed distinct episodes of foraging activities, including increased swim speeds indicative of prey pursuit 14 , and aggregation behavior following prey capture, presumably to share prey among group members, particularly between dependent offspring and mothers. A recent study utilized movement variables of subsurface behavior to identify prey capture events, in order to explore sex differences in foraging ecology of fish-eating killer whales, revealing males made more prey capture dives than females, presumably to support the energetic requirements of a larger body size 19 . Currently, however, little is known about the extent to which these prey pursuits represent the culmination of continuous search efforts, or whether foraging occurs in distinct periods in space and time, allowing for other activities including recovery following metabolically costly deep dives, socialization, traveling, or other non-foraging activities.…”

“…Currently, however, little is known about the extent to which these prey pursuits represent the culmination of continuous search efforts, or whether foraging occurs in distinct periods in space and time, allowing for other activities including recovery following metabolically costly deep dives, socialization, traveling, or other non-foraging activities. Additionally, given the sexual dimorphism between males and females and differences in metabolic rates, energetic requirements 12 and foraging rates 19 , it is possible that males and females may partition behavioral activities or timing to minimize energy use while maximizing gain and meeting other needs including care of dependent offspring. Indeed, in many species of sexually dimorphic marine mammals, foraging behavior is sexually segregated 20–22 .…”

A review of Canadian Arctic killer whale (Orcinus orca) ecology

“…Satellite-tracking studies should continue to facilitate an improved understanding of killer whale spatial ecology (e.g., movement, distribution, and habitat selection), allowing researchers to make predictions concerning understudied areas and the consequences of future climate warming. Furthermore, archival suction-cup tags should be used to facilitate an improved understanding of finer-scale spatial ecology (Tennessen et al 2019). Finally, a network of moored hydrophones should be established and used to identify and monitor killer whale occurrence (see Stafford 2019).…”